The long-term goal of this proposed research is to elucidate the mechanism by which anticonvulsant drugs activate CNS serotonergic neurons to produce an anticonvulsant effect. For the present proposal, the mechanism(s) by which carbamazepine enhances serotonin release and the role that this serotinin release plays in the suppression of convulsions will be investigated in two animal models. The effects of carbamazepine on serotonin release and on sound-induced convulsions will be evaluated in genetically epilepsy-prone rate (GEPRs) of the severe seizure strain (GEPR-9s) and of the moderate seizure strain (GEPR-3s). Similarly the effects of carbamazepine on serotonin release and on maximal electroshock seizures will be evaluated in Sprague-Dawley rats. The relationship between the anticonvulsant effect of carbamazepine, the effect of carbamazepine on extracellular serotonin and the brain tissue and brain extracellular fluid concentrations of carbamazepine and its active metabolite, carbamazepine-epoxide will be assessed with the aid of microdialysis techniques in the two GEPR strains as well as in Sprague- Dawley rats. The results of these studies will aid in the determination of whether differences in responsiveness to the anticonvulsant effect of carbamazepine between GEPR-3s, GEPR-9s and Sprague-Dawley rats are due to pharmacokinetic (distribution) or pharmacodynamic (innate responsiveness) factors. This difference in responsiveness also will be studied by delivering a range of concentrations of carbamazepine to specific brain areas via a microdialysis probe. During the administration of carbamazepine, the dose relatedness of the increase in extracellular serotonin will be assessed. In another series of experiments the overflow of serotonin produced by carbamazepine and carbamazepine-epoxide in superfused brain slices from the three strains will be evaluated. Finally, a series of studies designed to elucidate the cellular mechanism(s) by which carbamazepine increases extracellular serotonin will be carried out. These studies will be conducted both in vivo and in vitro. In the in vivo experiments, microdialysis techniques will be employed and brain areas will be dialyzed with artificial cerebrospinal fluid containing varying ionic compositions (e.g. zero calcium or high potassium solutions) and/or drug-tools (e.g. tetrodotoxin) in order to assess the neuronal processes by which carbamazepine increases serotonin release. In the in vitro experiments, perfused brain slices derived from brain areas rich in serotonergic terminals (e.g. the hippocampus) and from brain areas containing cell bodies as well as terminals (e.g. brain stem areas including the dorsal raphe nuclei) will be studied using established approached. These in vitro approaches will allow us to determine if carbamazepine increases extracellular serotonin via a neurochemical mechanism by which other drugs are known to increase neurotransmitters. The results of these studies should aid in out understanding of the mechanisms by which antiepileptic drugs suppress convulsions. In turn, this mechanistic information should aid in the design and development of improved drug treatments for epilepsy.